This invention relates to a method and a system for compression of the flow of data transmitted between a television transmitter and a television receiver.
In digital television, the luminance and color-difference signals are digitized in eight bits per point with a structure and a sampling frequency which depend on the problem to be solved. These data are usually transmitted in real time and the raw data flow rate is very high, namely in principle higher than 140 megabits/second, thus entailing the need to reduce the flow rate in order to permit the use of existing transmission media.
Different methods and devices for flow reduction are already known. Among these, the methods of coding television signals by modulation of differential coded pulses are particularly attractive by reason of their simplicity of application. They are all the more appreciable by virtue of the fact that the transmitted data are constituted by binary-code words of fixed length, since the problems of management of the buffer memories required for adapting the variable output of the transmitted to the fixed output of the channel which connects the transmitter to the receiver are removed. Furthermore, the use of intra-frame coding avoids the need to make systematic use of frame memories. The known differential coding methods consist in coding the difference between the value of a sample of the signal and an estimation, or prediction, computed from the values of the preceding samples already coded, this difference being quantized by a quantizer having n quantization levels. With each level i is associated a code C.sub.i which is transmitted over the line or transmission channel. The received code is converted to its real value which is then added to a prediction value computed by the receiver in order to reconstruct the signal. By means of a negative-feedback loop, a prediction can be made at the level of the transmitter and is identical with the prediction formulated at the receiver.
Examples of construction of devices for differential coding-decoding of digital data are described in French patent Application No. 81 20167 filed in the name of the present Applicant.
A problem arises, however, when it is sought to apply the methods of differential coding-decoding to transmission of television images, especially when it is desired to obtain a substantial compression of flow of transmitted data. The problem lies in the fact that it is not possible by means of differential coding methods to reduce to less than four bits per image point transmitted with intra-frame coding and a fixed-length code for the luminance component. Below this value, the quality of the reconstructed image is no longer acceptable.
A substantial compression of flow in fact results in spaced quantization levels, thus presenting problems of image reproduction, both in regard to the uniform image zones and in regard to the image contours. It is found that, in the uniform zones of the image, small variations in luminance are directly observed by the eye and that it would consequently be preferable to quantize the luminance signal of the uniform zones by means of quantizers having closely spaced levels in order to prevent excessive amplification of small luminance variations which might otherwise cause false contours to appear in the vicinity of the zero prediction error whereas, on the other hand, and in the case of the image contours which mark the transition between two uniform zones, quantization by means of widely spaced reconstruction levels would be preferable in order to provide better reproduction of the contours. However, in the second case just mentioned, the spacing between two levels cannot exceed a predetermined limit since the contours which appear beyond this limit are reproduced in the form of stair-steps.
In order to solve this problem, one solution consists in changing-over the quantizers to two different quantization characteristics as a function of the local appearance of the image point to be transmitted. For example, in the case of points located in uniform zones of the image, coding will be performed by means of a quantizer having closely-spaced reconstruction levels in the vicinity of the zero prediction error. In the case of points located in zones of image contour or texture, a quantizer having high reconstruction levels will be employed.
The known methods of changeover to different quantization characteristics can be placed in two categories, depending on whether the instant of changeover is transmitted or not.
If the indication of a change in quantization characteristic is not transmitted, the characteristic is obtained in exactly the same manner at the transmitter and at the receiver, from tests performed on image points which are already known at the receiver and constitute the causal neighborhood of the point to be coded. Under these conditions, if a fixed-length code is employed, the output of each image line is constant since the only information to be transmitted is the value of the quantized prediction error. In some cases, however, a problem arises when a causal neighborhood does not alone suffice to find the best characteristic of the quantizer to be employed. This is a particularly crucial problem when the points of the image to be quantized are located within a transition zone between a uniform zone and the zone of image contours.
On the other hand, if the quantization characteristic changeover takes place on points which are not known to the receiver and constitute a non-causal neighborhood for the receiver, the transmitter must necessarily provide the receiver with indications in regard to a change in characteristic. In this case, even if a fixedlength code is employed for coding quantized prediction errors, the output of each image line is variable and problems then arise in regard to the management of the buffer memories required in order to adapt the variable output of the transmitter to the fixed output of the transmission channel.